In order to inspect lithography masks or mask blanks, that is to say blanks for producing lithography masks, they have to be brought to their focus position in the metrology system. A focus stack of a specific test structure on the mask is usually recorded for this purpose. However, this is not always possible. A further disadvantage of the known methods is that the results are often not reliably reproducible. They may depend in particular on the test structure, the illumination setting and a more or less arbitrarily predefinable contrast criterion. Moreover, the customary methods do not function if coherent illumination settings with a low pupil filling coefficient are used for illuminating the mask.